The use of conducting polymer thin films as the active layers in optoelectronic devices has been growing in interest for the past decade. Development of conducting polymer-based devices such as LEDs, photodiodes, flat-panel displays, solar cells, lasers, and transistors has proceeded rapidly, and in many cases commercialization is imminent. However, a major drawback to the commercialization of all these types of devices is the rapid rate of photo-oxidation of the conducting polymer under ambient conditions, which in all cases degrades device performance and ultimately limits device lifetime.
The dynamics of the photo-oxidation process in conducting polymers has been studied extensively. In photo-oxidation, a primary, nonluminescent excitation in the conjugated polymer (the triplet exciton) interacts with oxygen diffused into the polymer film, transferring energy to the oxygen and forming a highly reactive excited state of oxygen (singlet oxygen), which chemically reacts with the conjugated polymer, forming exciton traps. These exciton traps are topological defects, chemically corresponding to carbonyl defects, added on polymer chains and chain scission, which provide an additional nonradiative relaxation channel for the polymer singlet excitons, thus quenching the polymer luminescence. Hence, once sufficient exciton traps have formed, the polymer is rendered inoperable for its intended purpose.
The most widely used method for improving conducting polymer-based device lifetime is encapsulation. The range of encapsulation methods available are limited to low temperatures due to degradation of the polymer active layer at temperatures approaching the melting point (generally less than 200° C.) of the polymer. Typical encapsulation methods for polymer-based devices include deposition of multiple organic or inorganic layers, which may be doped with oxygen scavengers, sandwiching the device between glass substrates bonded with epoxy, or a combination of the two. Devices using a combination of these techniques have demonstrated to have operating lifetimes in excess of 10,000 hours and storage lifetimes of at least 2 years, determined by accelerating testing conditions (elevated temperature and humidity). However, these techniques are limited to devices on rigid substrates. One possible advantage of conducting polymer-based devices over inorganic devices is the ability to fabricate devices on flexible substrates, allowing for simple mass production by reel-to-reel coating. To the inventors' knowledge, this advantage cannot readily be realized, because encapsulated flexible polymer devices are not yet commercially available.
Another method of protecting conducting polymer films against photo-oxidation is the addition of a stabilizer material to block the action of the oxygen. Several materials generally used to combat oxidation in polymers such as polyethylene have been studied in polythiophene devices, with no observable effect. For example, significant protection against photo-oxidation was afforded to polythiophene derivatives by the addition of 1-phenyldodecan-1-one (PDK). However, the additive (PDK) was shown to protect the polymer by absorbing UV light without transferring energy to the polymer. This protection is of no use in electroluminescent devices, which operate on the principle of electron-hole recombination, instead of photon absorption to form radiative species. Electron-hole recombination utilizes energy applied to electron-hole junctions to excite atoms and subsequently maintain light emission. Alternatively, the addition of C60 to polyphenylenevinylene derivatives has been shown to drastically reduce photo-oxidation of the polymer. Unfortunately, the C60 has the additional undesirable effect of efficiently quenching the luminescence of the polymer.
Because the desire to produce conducting polymer-based devices continues to grow, there exists a need to develop a method that improves the properties of the polymers used, namely by reducing photo-oxidation in the polymers without adversely affecting other properties of the polymers.